1) Install the Homebrew package manager for your simple OSS needs (wget, haskell, ..., whatever).

2) For more involved OSS needs, instead of messing the system with tons of Pythons, Rubies, RDBMSs, etc, or using MacPorts, I would suggest running those environments in a VM (VMWare Fusion or Parallels) running Linux. It could even be headless, and you could connect to it through the Terminal. You get a development environment similar to your production setup, snapshots, etc AND a clean base system.

3) For video watching: install some "codec pack" for Quicktime (it has few built-in codecs). Also install VLC and MPlayerX. For audio, use iTunes and pay some attention to proper tagging (if your mp3s don't have it already).

4) If you start buying programs now, use the Mac App Store. You get to install the bought programs in all your Macs (if you have more than one Mac at some point), and when you clean-install your OS or get a new Mac you'll be able to download and have them installed automatically from one place.

5) Download and install XCode (this is also used by Homebrew [1]), but it will be useful for you in general if you do any programming, as it has the C compilers, headers, etc. Also install the "Command line tools" (see comment below).

6) When looking for a solution for a OS X problem, a lot of people in forums will suggest "fixing permissions" and "zapping the PRAM". Those are, 99% of the time, BS cargo-cult non solutions.

7) You DON'T need an antivirus. Just exercise common caution. It's nothing like Windows XP of times past.

8) You really DON'T need to shutdown your OS X. Maybe once in a blue moon. Just close the lid and let it sleep.

9) All OSs have problems. Especially when you include the userland programs in the possible bug space. If CPU is pegged at 100%, open "Activity Monitor" and find the culprit program and kill it. More often than not, it's Flash.

10) Check the Applications/Utilities folder. Lots of useful stuff in there.

11) Learn about the "defaults" command. You can use it to enable many hidden options and customizations from the command line.

12) If you like fine-tuning and customizing, get familiar with the /Library and ~/Library folders. It's where a lot of programs keep plugins, themes, settings, etc. There is a defaults [11] command to make the ~/Library folder visible in the Finder (normally it's hidden from the average user).

13) Check the Automator for application automation (also: Applescript). You can a lot of stuff with it, if you like these kind of things.

14) Read Siracussa's Ars Technica Mountain Lion review. All of it. And maybe a book, like Mountain Lion, the Missing Manual. You'll learn a lot of things that you will otherwise pick up randomly after months of using OS X.

15) Some apps to check: Alfred (must have: launcher), Evernote (note taking), iA Writer (distraction free writing), Skitch (image capture / annotations, though current version is botched), Fission (barebones audio editing), Pixelmator (bitmap image editor), Reeder (RSS reader), Little Snitch (must have: application firewall), Tunnelblick (VPN), Transmit (FTP/SFTP/S3 etc), Unarchiver (compression archive handler), VMWare Fusion (virtual machine host), Sublime Text 2 (programmers' editor, if you're not into Emacs/Vim/IDEs). For simple multimedia needs, check the iLife programs. For work needs, iWork programs are quite capable, though not as much as MS Office. Adobe home user programs (Photoshop and Premier Elements) are available for the Mac too.

16) Avoid HAXIEs and APE (cross-OS hijacking system used to attach extension functionality to applications). Not very stable, and not really needed. ( http://en.wikipedia.org/wiki/Haxie ).

17) If you go with Apple's way, you get more integration between your OS X programs.

E.g. if you're using Mail.app instead of Thunderbird and iPhoto instead of some third party photo manager, then, while you write a new email, you can select pictures in your iPhoto library to attach, from a built-in image picker in Mail.app. Same for iLife and iWork apps. E.g you can select, through Keynote, a song from your iTunes library to be use in your presentation. Personally, I use mostly Apple's apps, but prefer Chrome to Safari and Lightroom to iPhoto/Aperture.

18) Some things that might give you problems, from time to time:
- Spotlight indexing (when you add lots of new stuff from an external disk to your primary disk, or when it's stuck on a corrupted file, etc). You can turn it off and on for specific disks and folders.
- Flash using 100% of the CPU. Kill it from activity monitor, or kill the tab.

19) I personally tend to prefer installing new major OS X versions from scratch. It's easy to port over your old files, mails, and settings from a backup. That said, updating also works fine. YMMV.

20) Use something like Carbon Copy Cloner or Super Duper! for bootable, incremental, backups of your startup disk. Time Machine backups are handy too, but not bootable.

21) Have fun.